Targeted delivery of drugs using water-soluble polymeric carriers improves efficacy and reduces toxicity. However, the large size of most biopolymers necessitates administration via the parenteral route. As oral absorption remains the preferred route of administration, many pharmacologically active drugs as well as polymers have poor systemic availability when administered orally. Our preliminary data indicate that poly(amidoamine) (PAMAM) dendrimers efficiently translocate across epithelial cell monolayers and can effectively increase the transfer of small molecules. Thus, this polymeric carrier may be used to improve the oral absorption of therapeutic molecules into the systemic circulation and direct them to the target sites via the creation of a conjugate delivery system. In the current proposal we aim to evaluate the influence of structural features of PAMAM dendrimers on their transepithelial transport and capitalize on both their permeation enhancing effects to increase the oral bioavailability and their macromolecular nature to facilitate targeting of a model drug. The long-term objective of this proposal is to develop polymeric systems based on PAMAM dendrimers for targeted oral drug delivery. The central hypothesis to our proposed research is that the permeation and targeting potential of poorly absorbable drugs is enhanced by conjugation to PAMAM dendrimers. To test our hypothesis, we will study the following Specific Aims: 1) To evaluate the influence of charge and drug loading on intestinal transport and cytotoxicity of PAMAM dendrimers. 2) To evaluate the in vitro stability and in situ bioavailability of dendrimer-drug conjugates. 3) To evaluate the in vivo bioavailability, antitumor efficacy and toxicity of the conjugates in a murine model of liver metastasis of colorectal cancer. 4) To delineate the intestinal transport mechanism(s) of PAMAM dendrimers. By evaluating the influence of structural features on permeability and stability we expect to find prototype PAMAM dendrimers that can increase oral availability of SN38, exhibit minimal toxicity, and target the drug effectively to liver metastases. Delineation of the mechanism(s) of transport will aid in the rationale design of novel polymeric carriers for oral delivery. In the long term this may allow for the delivery of a variety of therapeutic molecules in a more controlled and targeted manner across the Gl tract using tailor-made PAMAM dendrimers as polymeric carriers.